Perfluorocarbon (PFC), nitrogen trifluoride (NF3), and the like have been used as cleaning gas in CVD devices (chemical vapor deposition method) used in a semiconductor production process or a liquid crystal production process, and in particular, in main processes of formation of oxide films or insulator films. The aforementioned two types of gases have an ozone depletion potential (ODP) that is zero. However, such gases have a global warming potential (GWP) for 100 years that is approximately 10,000 times greater than that of CO2 (carbon dioxide). Thus, it has been desired that some type of alternative technique or alternative material be developed.
It is to be noted that such an ozone depletion potential (ODP) indicates the ability of each substance to destroy ozone, which is integrated with an infinite time, based on the present findings. The ODP is represented by a coefficient (relative value) obtained when chlorotrifluoromethane is defined as 1.
In addition, such a global warming potential (GWP) indicates the influence of each substance upon global warming, which is integrated with a certain period of time (100 years) per unit mass. The GWP is represented by a coefficient (relative value) obtained when carbon dioxide is defined as 1.
In order to develop an alternative technique or material that is substitutable for CVD chamber cleaning gas such as PFC or NF3, a national project has advanced, and carbonyl fluoride (COF2) has been selected as a dominant alternative material (refer to Non-Patent Document 1).
Carbonyl fluoride (COF2) is nonflammable gas having a molecular weight of 66 and a boiling point of −83° C. This gas has excellent cleaning properties, and it has the same level of cleaning effect when compared with conventional products. In addition, if such carbonyl fluoride coexists with water, it is decomposed to CO2 (carbon dioxide). Since COF2 is easily decomposed in the ambient air, a GWP value of 100 years is extremely small. Even an indirect GWP value of 100 years, which involves CO2 generated as a result of decomposition, is 0.7 or less. Accordingly, when compared with PFC or NF3, whose GWP value is considered to be 10,000 times greater than that of CO2 (carbon dioxide), carbonyl fluoride is overwhelmingly advantageous. In the aforementioned project, it has been reported that COF2 has the effect of significantly reducing a greenhouse effect, when compared with the aforementioned cleaning gases (refer to Non-Patent Document 2, for example).
Moreover, in the liquid crystal industry, practical application of COF2 has already been reported. Since COF2 is easily eliminated by a water scrubber, devices for eliminating NF3 or PFC, which have been conventionally required, are not required. Thus, it is considered that practical application of COF2 contributes to a reduction in energy consumption in production processes (refer to Non-Patent Document 3, for example).
In general, methods for synthesizing such COF2 are broadly divided into four known methods: a method of allowing carbon monoxide or carbon dioxide to react with a suitable fluorinating agent such as fluorine or silver difluoride for oxidation; a method of allowing carbonyl dichloride, namely, phosgene to react with a suitable fluorinating agent such as hydrogen fluoride, antimonyl trifluoride, arsenic trifluoride, or sodium fluoride, so as to carry out halogen exchange from chlorine atoms in molecules to the corresponding fluorine atoms; a method of allowing trifluoromethane to react with oxygen; and a method of allowing tetrafluoroethylene gas to react with oxygen.
Specific examples of these methods will be described below.
1. Methods Using Carbon Monoxide or Carbon Dioxide as Raw Material;
The following methods have been known as methods using carbon monoxide or carbon dioxide as a raw material, for example:
(a) a method of directly fluorinating carbonic acid gas and fluorine gas in a gas phase (refer to Patent Document 1);
(b) a method involving electrolytic fluorination of carbon monoxide (refer to Patent Document 2);
(c) a method comprising adding at least one type of third component gas selected from among N2, He, Ne and Ar, when carbonyl fluoride is continuously produced by the reaction of carbon monoxide with fluorine gas, then carrying out the reaction while supplying the gas in a dynamic state and under a reduced pressure, and then circulating the third component gas or the third component gas containing unreacted carbon monoxide after the carbon fluoride has been captured with a cold trap (refer to Patent Document 3);(d) a method of allowing a fluorine-containing compound such as a metal fluoride that is in a state excited by plasma to react with CO, so as to obtain a gaseous reaction mixture, and then quenching the gaseous reaction mixture to obtain COF2, thereby producing carbonyl fluoride (refer to Patent Document 4); and(e) a method of directly fluorinating carbon monoxide with fluorine gas (refer to Non-Patent Document 4, for example).
However, such methods involving electrolytic fluorination or direct fluorination of carbon monoxide are not industrially adequate because they require an expensive electrolytic cell or a corrosive-resistant material, or because large equipment is required for regulating a large amount of heat of reaction. In addition, in a method of synthesizing COF2 by a direct reaction between carbon monoxide and fluorine, since it is a reaction between carbon monoxide that is inflammable gas and fluorine that is strongly oxidizer gas, such a reaction may explosively occur or impurities such as carbon tetrafluoride may be generated due to the heat of reaction, thereby resulting in a decrease in purity.
2. Methods Using Phosgene as Raw Material;
The following methods have been known as methods using phosgene as a raw material, for example:
(a) a method of blowing phosgene into a hydrogen fluoride aqueous solution to which triethylamine has been added, or a hydrogen fluoride aqueous solution into which an alkali metal fluoride has been dispersed (refer to Patent Document 5);
(b) a method of fluorinating phosgene with sodium fluoride in a solvent (refer to Patent Document 6);
(c) a method of fluorinating phosgene with hydrogen fluoride via an activated carbon catalyst in a gas phase (refer to Patent Document 7); and
(d) a method comprising allowing phosgene to come into contact with an inorganic fluoride in a gas phase, then allowing the resultant to come into contact with activated carbon in a gas phase to obtain phosgene and difluorocarbonyl chloride, and allowing them to come into contact with activated carbon in a gas phase, so as to obtain carbonyl fluoride (refer to Patent Document 8).
In such reactions using phosgene, however, it is necessary to use highly toxic phosgene as a raw material. Further, the synthesized COF2 contains impurities such as carbonyl chloride fluoride derived from chlorine or a fluorinating agent. Furthermore, it is difficult to separate such COF2 from carbon dioxide derived from water. Thus, the aforementioned methods have not necessarily been satisfactory.
For example, method (a), which involves fluorination with hydrogen fluoride in the presence of a solvent, is disadvantageous in terms of difficulty in separation of COF2 from the generated hydrogen chloride. In the aforementioned method, which involves fluorination of phosgene with hydrogen fluoride in the presence of a solvent and triethylamine or fluorination of phosgene with sodium fluoride in the presence of a solvent, carbonyl fluoride can be obtained without generation of hydrogen chloride. However, since hydrochloride of triethylamine or sodium chloride is generated in an equimolar amount of the generated carbonyl fluoride, the disposal or recycling thereof becomes necessary.
In method (c) involving fluorination of phosgene with hydrogen fluoride via an activated carbon catalyst, carbonyl fluoride is mainly generated under the aforementioned conditions, and it becomes difficult to eliminate hydrogen chloride generated as a by-product.
In method (d), which comprises fluorinating phosgene with an inorganic fluoride in a gas phase to obtain a mixture containing difluorocarbonyl chloride, allowing the mixture to come into contact with activated carbon to convert it to a mixture of difluorocarbonyl chloride and phosgene, and disproportionating the difluorocarbonyl chloride with an activated carbon catalyst, so as to obtain carbonyl fluoride, carbon monoxide and chlorine are generated as by-products during the fluorination of phosgene with an organic fluoride. In order to convert such by-products to phosgene, it is necessary to allow them to come into contact with activated carbon. Thus, extra equipment is required.
3. Method Using Trifluoromethane as Raw Material;
As a method using trifluoromethane as a raw material, a method of allowing trifluoromethane to react with oxygen under heating has been known, for example (refer to Patent Document 13).
However, in order to favorably produce carbonyl fluoride by this method, a high reaction temperature of 500° C. or higher is necessary. In addition, there are many cases where carbon dioxide is contained as a by-product in the reaction product. It is difficult to eliminate such carbon dioxide from carbonyl fluoride.
4. Methods Using Tetrafluoroethylene as Raw Material;
The following methods have been known as methods using tetrafluoroethylene as a raw material, for example:
(a) a method of allowing ethane fluoride to react with ozone (refer to Non-Patent Document 5, for example);
(b) a method of allowing tetrafluoroethylene (TFE) to react with oxygen containing oxygen difluoride to obtain carbonyl fluoride (refer to Patent Document 9); and
(c) a method of oxidizing tetrafluoroethylene (TFE) with oxygen in an equimolar amount of TFE in coexistence with a large amount of diluent comprising a fluorine compound, so as to obtain COF2 (refer to Patent Documents 10 and 11, for example).
However, in the reaction of ethane fluoride with ozone described in (a) above or in the static reaction of carbon monoxide with carbon tetrafluoride, both the yield and purity of the product are low, and thus it is difficult to use such methods as industrial processes.
In method (b), generation of carbonic acid gas has not been confirmed. However, since carbonyl fluoride is easily hydrolyzed by water contained in a raw material or a catalyst to generate carbonic acid gas and hydrogen fluoride, there is also a possibility that carbonic acid gas is generated after completion of the reaction. In particular, the amount of carbonic acid gas generated as a by-product or mixed into the reaction product as an unreacted raw material makes up several percent to several tens of percent of the amount of the reaction product. For the intended use of COF2 as cleaning gas in production of semiconductors, reduction in the amount of such carbonic acid gas or separation of such carbonic acid gas is required. In addition, since oxygen difluoride used as a raw material is an explosive substance, there is a high risk in operating it.
Moreover, it is necessary for method (c) to carry out the reaction in a flow system using TFE and oxygen in an equimolar amount of the TFE at a temperature between 200° C. and 450° C. for a reaction time between 1 and 10 seconds, also using, as a diluent, carbonyl fluoride, Freon-22, Freon-113, Freon C51-12, or FC-75 comprising a mixture of perfluoro cyclic ethers, at a molar ratio between 10:1 and 100:1 with respect to the oxygen. This publication has reported that a large amount of heat of reaction is generated in the reaction of TFE with oxygen, and that if a mixture obtained by mixing TFE with oxygen in an equimolar amount of the TFE is generally heated, it is exploded, and thereby only a small amount of COF2 is obtained. Thus, a diluent is used to suppress such explosion according to the aforementioned publication. However, a fluorine compound used as such a diluent is not necessarily inexpensive, and further, such a diluent has also been problematic in terms of an operation to separate the diluent from the product.
Furthermore, TFE used as a raw material in production of COF2 is produced by applying a high-temperature thermal decomposition method to monochlorodifluoromethane in the industrial field. Originally, such a thermal decomposition reaction is a complicated reaction attended with decomposition of molecules and the recombination thereof. Thus, many types of by-products are generated in this reaction. Accordingly, it is an important object to efficiently separate and purify a high-purity TFE of interest from a product generated as a result of the thermal decomposition, which comprises many types of by-products, and to efficiently reuse unreacted R-22 (refer to Patent Document 12).
5. Method for Producing TFE Used as Raw Material in Production of COF2;
Tetrafluoroethylene (TFE) can be produced by applying a high-temperature thermal decomposition method to chlorodifluoromethane HCFC-22 (which is also referred to as “R-22” at times) in the industrial field.
As a thermal decomposition temperature applied in the thermal decomposition reaction of HCFC-22, any temperature may be applied, as long as it is a temperature at which HCFC-22 can be decomposed. In order to enhance the yield of TFE, the reaction is generally carried out in an atmosphere at a temperature between 600° C. and 700° C. There are two thermal decomposition methods, namely, an external thermal decomposition method and an internal thermal decomposition method. The internal thermal decomposition method is a method of allowing HCFC-22 to come into contact with heated water vapor for heating the HCFC-22. This method is adopted in many field sites. On the other hand, the external thermal decomposition method is a method comprising supplying HCFC-22 to a thermal decomposition reaction device and then providing heat thereto from outside of the reaction device by a method such as use of a heating medium or direct heating.
By the way, since such a thermal decomposition reaction is a complicated reaction attended with decomposition of molecules and the recombination thereof, many reaction by-products are generated as a result of this reaction. Accordingly, it has been an important object to efficiently separate and purify high-purity TFE of interest from thermally decomposed products containing such reaction by-products, or to efficiently reuse unreacted HCFC-22.
For the aforementioned object, TFE has been purified by a method comprising: cooling, deoxidizing and drying the product obtained by thermal decomposition of HCFC-22; supplying the resultant to a first rectification device for rectification; discharging the total quantities of components such as carbon monoxide or trifluoromethane whose boiling point is lower than that of TFE from the top of the first rectification device, and at the same time, eliminating components including TFE other than the aforementioned components, which have a high boiling point, from the bottom of the aforementioned device; and then supplying such components having a high boiling point to a second rectification device, so as to distill the TFE of interest from the top thereof and eliminate components having a higher boiling point than that of the TFE from the bottom thereof.
At that time, the components eliminated from the bottom of the second rectification device comprise many components having a higher boiling point than that of TFE, as well as HCFC-22 used as a raw material. Thus, the components eliminated from the aforementioned bottom have been purified to separate HCFC-22, and thereafter, R-22, which had been consumed by thermal decomposition, has been replenished and has been then supplied to the thermal decomposition process again.    [Patent Document 1] Japanese Patent Laid-Open No. 11-116216    [Patent Document 2] Japanese Patent Publication No. 45-26611    [Patent Document 3] Japanese Patent Laid-Open No. 2003-267712    [Patent Document 4] National Publication of International Patent Application No. 2002-515011    [Patent Document 5] Japanese Patent Laid-Open No. 54-158396    [Patent Document 6] U.S. Pat. No. 3,088,975    [Patent Document 7] U.S. Pat. No. 2,836,622    [Patent Document 8] E.P. Patent No. 0253527    [Patent Document 9] U.S. Pat. No. 3,639,429    [Patent Document 10] U.S. Pat. No. 3,404,180    [Patent Document 11] USSR Inventor's Certificate No. 424809 (1974) and RU (11) 2167812    [Patent Document 12] Japanese Patent Laid-Open No. 7-233104    [Patent Document 13] International Publication WO2005/105668    [Non-Patent Document 1] “Evaluation of COF2 in Mass Production Line,” Masaji Sakamura, and “Alternate Gas for CVD Cleaning,” Yuki Misui, 12th Annual ISESH Conference (Portland) Jul. 19-23, 2005    [Non-Patent Document 2] Internet <URL: http://www.rite.or.jp/Japanese/kicho/kikaku/world/world04/01-18—19.pdf> Semiconductor CVD Cleaning Project Report [searched in Aug. 16, 2005], Technical Information Magazine “RITE WORLD” No. 1 (first issue) Study Group News, Planning Section, Planning and Research Group, the Research Institute of Innovative Technology for the Earth (RITE)    [Non-Patent Document 3] “COF2 used as cleaning gas in production of liquid crystal,” Nikkei Sangyo Shimbun, Jun. 30, 2005    [Non-Patent Document 4] J. Amer. Chem. Soc., 91, 4432 (1969)    [Non-Patent Document 5] J. Amer. Chem. Soc., 102, 7572 (1980)
As stated above, methods for producing carbonyl fluoride (COF2) are broadly divided into 4 methods. One of the 4 methods, namely, a method using tetrafluoroethylene (TFE) as a raw material described in the aforementioned Patent Document 10 has required addition of an expensive gaseous fluorine compound (diluent) in an amount 10 to 100 times larger than oxygen, in order to avoid explosion.
It has been known that such TFE used as a raw material is produced from HCFC-22. However, since this production method involves thermal decomposition, various types of by-products comprising a fluorine compound as a main body, such as trifluoromethane, are generated. Thus, in order to purify the reaction product, and also in order to recover unreacted HCFC-22, enormous manpower, cost, and equipment have been required.